Field of the Invention
The invention relates to a thermal power plant, including a steam turbine having a turbine rotor directed along a main axis and surrounded by an inner housing. A guide-blade structure, which surrounds the turbine rotor in the circumferential direction and has guide blades, is disposed in the inner housing. The invention also relates to a method for cooling a steam turbine in the ventilation mode, in particular a low-pressure steam turbine.
It is known, for example from a book entitled "Stromungs-maschinen" [Turbo-Machines] by K. Menny, Teubner-Verlag, Stuttgart, 1985, Section 3.4.6"Na.beta.dampfstufen" [Wet-Steam Stages], that condensation of action steam takes place in steam turbines, in particular in so-called wet-steam stages. During an expansion of the steam in the steam turbine, supercooled steam occurs if there is a fall below a boundary curve with the wet-steam region, for example in the case of condensing turbines. The temperature of the supercooled steam is lower than a saturation temperature associated with the steam point. At specific supercooling, spontaneous condensation commences, in which small mist droplets occur that may settle on guide blades in the form of a water film or individual strands of water. The water film breaks away from trailing edges of the guide blades and forms secondary drops having a diameter of up to about 400 .mu.m. Those steam droplets which break away may lead to a stripping of material, if they impinge on the moving blades, particularly when the drops have a diameter on the order of magnitude of 50 to 400 .mu.m (so-called drop impact erosion). In order to avoid such drop impact erosion, the water film is often sucked away directly on the guide-blade surface. For that purpose, a hollow guide blade has slots which connect its interior to the condenser of the steam turbine.
German Published, Non-Prosecuted Patent Application DE-OS 19 51 922 specifies a device for preventing the formation of droplets in the low-pressure stages of steam turbines. Droplets are prevented from forming by feeding hot steam to the guide blades of the last guide-blade rows through an outer ring. The hot steam is conducted through the hollow guide blades to an inner ring and is conducted out of it again through a geodetically low-lying outflow conduit. The guide blades are to be heated to such an extent by feeding hot steam that condensation cannot take place at all.
Austrian Patent 250 402 describes introducing steam from preceding stages into guide-blades and feeding it into the steam flow again through slots in the guide blades. The avoidance of the formation of condensate on guide blades is likewise dealt with in U.S. Pat. No. 3,306,576, wherein hot steam is fed to a hollow guide blade and passing out of it through bores into the steam flow. The hot steam heats the steam flow to such an extent that the saturation temperature is exceeded at least locally and no condensation takes place.
A steam turbine blade which has a hollow structure and has an orifice for diverting steam into a main steam flow, is likewise described in the abstract of Japanese Patent Application 54-14 1908, Patent Abstracts of Japan, Jan. 18, 1980, Vol. No. 4.
European Patent 0 602 040 B1, corresponding to German Published, Non-Prosecuted Patent Application DE 41 29 518 A1and corresponding U.S. Pat. No. 5,490,386, describes a method for cooling a low-pressure steam turbine in the ventilation mode, wherein the rotor of the steam turbine is rotated, without being subjected to steam to be expanded. In a low-pressure turbine working in the ventilation mode, a steam atmosphere prevails, having a static pressure which corresponds to the pressure prevailing in the condenser connected to the low-pressure turbine. The friction of the turbine blades on the steam (ventilation) may lead to a considerable amount of heat being generated, with the result that the turbine may be heated to a high, possibly even inadmissibly high temperature. In order to avoid that from occurring, cooling measures are employed, in which, for example, condensate is injected, while at the same time being atomized, into the outlet or, if the cooling capacity to be applied has to be particularly high, into the inlet of the turbine. The condensate evaporates, with its temperature thereby being lowered, and as a result, the ventilating turbine is cooled. If injection takes place at the outlet, the cooling effect is often restricted to parts of the turbine in the vicinity of the outlet. If injection takes place at the inlet, condensate which agglomerates in the region of the inlet may put the blading of the turbine at risk due to surging. Therefore, according to European Patent 0 602 040 B1, corresponding to German Published, Non-Prosecuted Patent Application DE 41 29 518 A1 and corresponding U.S. Pat. No. 5,490,386, steam is fed into the steam turbine through a tapping point located between the outlet and the inlet of the steam turbine. Cooling in the turbine thereby first benefits the radially outer ends of the blades. The ends are subjected to the highest load as a result of the friction on the steam located in the turbine. The cooling effect is thus restricted essentially to those regions of the turbine in which it is desired. The cooling of other components of the turbine, for example, of the turbine shaft, is avoided.
Besides steam, condensate is additionally fed to a tapping conduit connected to the tapping point, in particular by injecting condensate into the steam transfer conduit and/or into the tapping conduit through the use of a condensate transfer conduit. The condensate is preferably mixed with the steam in an atomizer nozzle and is injected from that atomizer nozzle into the tapping conduit. A particularly high cooling effect is achieved by a condensate which is distributed into fine droplets and the droplet diameters of which are smaller than about 0.1 mm. The cooling process is controlled through a temperature measuring point located between the tapping point and the outlet, the feed of the steam or the feed of the steam/condensate mixture for tapping being regulated as a function of the measured temperature. The quantity of steam or steam/condensate mixture fed to the tapping conduit is approximately on the order of magnitude of 1% of the steam stream when the steam turbine is operating in the power mode. The steam used for cooling comes from a condensate container which serves for collecting, heating and degassing the condensate. Steam from the condensate container, to which hot steam is usually fed for the purpose of degassing the condensate, is saturated due to the coexistence of steam. Condensate, if appropriate, is even mixed with finely distributed condensate, and is therefore particularly suitable for injection into the ventilating turbine. Furthermore, steam may be extracted from a steam discharge conduit, through the use of which the steam is conducted past the low-pressure turbine in the ventilation mode. Such a steam discharge conduit conducts the steam, for example, from a high-pressure steam turbine preceding the low-pressure steam turbine or from a configuration formed of a high-pressure steam turbine and of a medium-pressure steam turbine, around the low-pressure steam turbine, to a heating device or the like, where the steam is possibly cooled and condensed. In order to obtain a steam/condensate mixture, the steam to be fed to the tapping point may be extracted from such a heating device. The steam may likewise be extracted from a high-pressure or medium-pressure steam turbine preceding the low-pressure steam turbine, directly or indirectly, for example from a preheater or the like fed by the turbine. Such steam normally has a sufficiently high characteristic pressure, so that feeding into the ventilating steam turbine can take place without separate pumps or the like.